Multiple axis multipoint non-contact measurement system

ABSTRACT

Apparatus for optically measuring a remote object comprises at least one source for projecting a plurality of discrete zones of electromagnetic radiation (for example laser spots) along a projection plane. Imaging apparatus images a plurality of reflections from the remote object. By spatially offsetting the imaging apparatus from the projection plane, easier discrimination and identification of the reflections is made possible.

FIELD OF THE INVENTION

This invention relates to triangulation-based non-contact measurement.In particular, this invention relates to multipoint triangulation-basednon-contact measurement wherein a plurality of light sources arereflected and imaged on an array.

BACKGROUND OF THE INVENTION

In multipoint triangulation-based sensors, a series of aligned spotsreflected from a workpiece are imaged by a camera that lies in the sameplane as both the spots and the light sources.

The camera typically comprises a two-dimensional pixel array onto whichthe spots are imaged. The range to each of the spots is derived bytriangulation from the location of its image on the array.

As was discussed in U.S. Pat. No. 5,986,745 to Hermary et al., a knownlimitation of such systems is the difficulty in sorting ordiscriminating between adjacent spots, for example as a result of theproximity or overlap of imaged spots, or due to dropout of a spot byocclusion from irregular surface characteristics.

One approach to enhancing reliable discrimination between spots is tomore accurately determine the centroids of the spots along thetriangulation axis. U.S. Pat. No. 5,056,922 to Cielo et al. disclosesthe use of an elliptical spot (e.g. the shape characteristic of TEM01mode laser light) wherein the longitudinal axis of the elliptical spotis perpendicular to the longitudinal axis of the detecting elements of alinear array. This allows more accurate determination of the centroid ofthe spots and hence, better discrimination between adjacent spots.

U.S. Pat. No. 5,986,745 to Hermary et al. addresses the problem by usinga spatially encoded pattern of light projected onto a workpiece ratherthan discrete spots.

U.S. Pat. No. 4,937,445 to Leong teaches the use of a small number ofbeams, so as to effectively limit the possibility of misidentificationof a given spot, while U.S. Pat. No. 4,687,325 to Corby relies on atime-separated series of different patterns of beams.

It is apparent that the problem of spot discrimination in multipointtriangulation sensing systems is well known and that a variety ofapproaches are used to address it.

The object of the present invention is to provide an improved, elegantand simple means of discriminating between successive spots in amultipoint triangulation-based sensor.

These and other objects of the invention will be better understood byreference to the detailed description of the preferred embodiment whichfollows.

SUMMARY OF THE INVENTION

In known prior art multipoint triangulation systems, the camera, the setof projected spots and the light sources are all in the same plane suchthat single reference-axis triangulation takes place.

According to the invention, the spots are imaged onto a two-dimensionalarray and the optical and camera set up is selected so as to providetriangulation of the spots effectively about two axes. Preferably oneaxis is shorter than the other.

The location of successive spots imaged from a workpiece onto the arraywill vary less along the shorter triangulation axis than along thelonger triangulation axis. Appropriate optical parameters for theshorter triangulation can be selected such that even with the maximumexpected variation in height of the workpiece, the spots as imaged alongthe shorter triangulation axis will be constrained to defined,non-overlapping areas of the array. Thus spot discrimination issignificantly improved. The longer triangulation axis is used fordimensional measurement while a combination of the shorter and longertriangulation axes is used for spot discrimination.

The inventors have also found that the foregoing can be achieved byspatially offsetting the camera from the plane in which all of thealigned spots lie. This results in range triangulation about the longeraxis and spot discrimination about a combination of the longer axis andthe axis defined by the spatial offset.

A look-up table may be used to compare the predetermined possiblepositions of each spot with the actual position of each reflection andthereby correctly associate each reflection with a given projected spot.

In one aspect the invention comprises optical measuring apparatuscomprising at least one source for projecting a plurality of discretezones of electromagnetic radiation along a projection plane and imagingapparatus for imaging a plurality of reflections of said zones from atleast one surface extending through said projection plane, said imagingapparatus being spatially offset from said projection plane. In a moreparticular aspect, the optical measuring apparatus comprises opticaltriangulation apparatus, the source comprises a laser and the zones ofelectromagnetic radiation comprise spots of light.

In another aspect, the invention comprises optical measuring apparatusfor determining spatial or dimensional characteristics of at least oneremote object, comprising a plurality of sources for projecting along aprojection plane a plurality of discrete zones of light onto said atleast one remote object, imaging apparatus for acquiring an image of aplurality of reflections of said zones of light from said at least oneremote object, said imaging apparatus being spatially offset from saidprojection plane, and a processor for outputting from said imagingapparatus data characterizing said image of said plurality ofreflections.

In a further aspect of the invention, there is also provided a record ofthe possible locations of each reflection in the captured image.

In yet another aspect, the invention comprises a method of opticallydetermining the spatial or dimensional characteristics of at least oneremote object, comprising projecting onto said object a plurality ofdiscrete zones of electromagnetic radiation along a projection plane,said projection plane intersecting at least one surface of said objectand acquiring an image of a plurality of reflections of said zones fromsaid at least one remote object from a vantage point that is spatiallyoffset from said projection plane.

In a further aspect, the method of the invention comprisespredetermining a range of possible locations in said image of each ofsaid plurality of reflections, and associating each one of a pluralityof said reflections with one of said zones of electromagnetic radiationby reference to the actual location of each of said reflections in saidimage and said possible locations of each of said reflections.

In yet a further aspect, the invention uses a two-dimensional array toacquire the image of the reflected spots.

The foregoing was intended as a broad summary only and of only some ofthe aspects of the invention. It was not intended to define the limitsor requirements of the invention. Other aspects of the invention will beappreciated by reference to the detailed description of the preferredembodiment and to the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described by reference to the detailed descriptionof the preferred embodiment and to the drawings thereof in which:

FIG. 1 a is a diagram of a typical optical set up according to the priorart;

FIG. 1 b is a view along line 1 b-1 b of FIG. 1 a;

FIG. 2 is a diagram of a two-dimensional array imaging the spots fromthe optical set up of FIGS. 1 a and 1 b;

FIG. 3 a is an optical set up according to the preferred embodiment ofthe invention;

FIG. 3 b is a view along line 3 b-3 b of FIG. 3 a showing the offset ofthe camera from the projection plane;

FIG. 4 is a two dimensional array for imaging the spots from the opticalset up of the FIGS. 3 a and 3 b; and,

FIG. 5 is an illustration of a lookup table according to the preferredembodiment.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

FIGS. 1 a and 1 b illustrate an optical set up according to aconventional prior art approach. A plurality of sources ofelectromagnetic radiation such a laser light sources 10 are aligned andarranged to project beams of light 12, 14 onto a workpiece (locatedgenerally in zone 16) along a common projection plane 18. It will beappreciated that at least one surface of the workpiece intersects theprojection plane 18.

Depending on the distance of specific surface points of the workpiece 16from the light sources 10, beam 12 may be reflected on the workpiece atpoints 12 a (short range) or 12 b (longer range) and beam 14 may bereflected at points 14 a or 14 b.

Camera 20 also lies in plane 18 as will be appreciated by reference toFIG. 1 b from which view the laser sources 10 and the camera 20 overlapwithin the plane 18. Referring to FIG. 2, the reflections of spots 12 aor 12 b and 14 a or 14 b are imaged onto a two-dimensional array 22 andthe distance between the imaged reflections and the light source isdetermined by the position of the reflections along the horizontal axisof the array. In the event that the reflections are 12 a and 14 b, theimaged reflections may be closely adjacent in array 22 or overlap asseen in FIG. 2, thereby inducing difficulties in sorting the reflectionsand associating them with the correct projected spots.

According to the preferred embodiment of the invention illustrated inFIGS. 2 a and 2 b, the optical set up of FIGS. 1 a and 1 b is modifiedby spatially offsetting camera 26 from plane 18 by a distance d. As aresult of the offset, triangulation is effectively accomplished abouttwo axes, thus resulting in relative displacement of successivereflections imaged on the array 28 along both the horizontal andvertical axes as shown in FIG. 4. Discrimination between, for example,points 12 a and 14 b therefore becomes significantly easier.

Dual axis triangulation in the preferred optical set up described aboveresults in different behaviour of the reflections on the imaging arraythan is the case with prior art optical set ups. As the range to thespot on the workpiece varies, the corresponding reflection is displacedacross the array in a generally diagonal path rather than along thehorizontal axis.

FIG. 5 illustrates the areas of possible locations for each reflectedspot as predetermined and recorded in a lookup table. For example, thearea defined by paralleliped 32 represents the possible locations for agiven reflected spot on the array. A record of the areas of possiblelocations for each reflected spot is stored by means of the lookuptable. In use, a processor assesses both row and column centroids foreach reflection and such centroids are used to determine range and toassign spot identity. The processor may also output data characterizingthe image.

In the preferred embodiment shown in FIG. 5, the areas of possiblelocation of the reflections as mapped onto the array are selected to belaterally contiguous. As a result, the identification of a reflection asbelonging to one or the other of the areas can be assessed by aprobability function. A reflection in the lateral center of an area canbe assigned a 100% probability of being associated with that area whilethe probability can decrease to zero as the location of the reflectedspot crosses an area boundary.

While the preferred embodiment contemplates a plurality of separatelight sources projecting along a common light projection plane, this isnot essential to the invention. Provided that the light spots reflectedfrom the workpiece are aligned with one another, the invention willenhance discrimination between the spots. The invention is thereforeapplicable to beam splitting configurations or configurations where thecameras are not necessarily disposed to project light along commonplanes.

It is also within the scope of the invention to use more than a singlecamera. A first camera could be used to provide an image usedprincipally for range determination of the various spots, while a secondcamera could be offset from the plane in which the spots lie for thepurposes of acquiring an image used primarily for spot discrimination.In such case, it would be necessary to associate the data from thesecond camera with the data from the first camera in order to correlatethe identity of the sorted spots to the spots for which range has beendetermined.

It will be appreciated by those skilled in the art that the preferredand alternative embodiments have been described in some detail but thatcertain modifications may be practiced without departing from theprinciples of the invention.

1. Optical measuring apparatus comprising: at least one source forprojecting a plurality of discrete zones of electromagnetic radiationalong a projection plane; and, imaging apparatus for imaging a pluralityof reflections of said zones from at least one surface extending throughsaid projection plane, said imaging apparatus being spatially offsetfrom said projection plane.
 2. Optical measuring apparatus according toclaim 1 wherein said optical measuring apparatus comprises opticaltriangulation apparatus, said source comprises a laser and said zones ofelectromagnetic radiation comprise spots of light.
 3. Optical measuringapparatus comprising: means for projecting a plurality of discrete zonesof electromagnetic radiation along a projection plane; imaging apparatusfor imaging a plurality of reflections of said zones from at least oneremote surface, said imaging apparatus having a point of view that isspatially offset from said projection plane.
 4. Optical measuringapparatus according to claim 3 wherein said optical measuring apparatuscomprises optical triangulation apparatus, said means for projectingcomprises a laser and said zones of electromagnetic radiation comprisespots of light.
 5. Optical measuring apparatus for determining spatialor dimensional characteristics of at least one remote object,comprising: a plurality of sources for projecting along a projectionplane a plurality of discrete zones of light onto said at least oneremote object; imaging apparatus for acquiring an image of a pluralityof reflections of said zones of light from said at least one remoteobject, said imaging apparatus being spatially offset from saidprojection plane; and, a processor for outputting from said imagingapparatus data characterizing said image of said plurality ofreflections.
 6. The apparatus of claim 5 further comprising a record ofthe possible locations of said reflections in said image.
 7. Theapparatus of claim 5 or 6 further comprising a processor for associatingeach of said reflections with one of said zones of light.
 8. Theapparatus of claim 6 further comprising a processor for associating eachof said reflections with one of said zones of light by reference to saidpossible locations.
 9. The apparatus of claim 5, 6 or 8 wherein saidoptical measuring apparatus comprises optical triangulation apparatus,said sources comprise at least one laser, and said zones of lightcomprise spots of light.
 10. A method of optically determining thespatial or dimensional characteristics of at least one remote object,comprising: projecting onto said object a plurality of discrete zones ofelectromagnetic radiation along a projection plane, said projectionplane intersecting at least one surface of said object; and, acquiringan image of a plurality of reflections of said zones from said at leastone remote object from a vantage point that is spatially offset fromsaid projection plane.
 11. The method of claim 10 further comprisingpredetermining a range of possible locations in said image of each ofsaid plurality of reflections.
 12. The method of claim 11 furthercomprising associating each one of a plurality of said reflections withone of said zones of electromagnetic radiation by reference to theactual location of each of said reflections in said image and saidpossible locations of each of said reflections.
 13. The method of claim10, 11 or 12 further comprising applying an optical triangulationtechnique for assessing said spatial or dimensional characteristics. 14.The method of claim 10, 11 or 12 wherein said step of acquiring an imagecomprises acquiring an image on a two-dimensional array.
 15. Opticalmeasuring apparatus comprising: at least one source for projecting aplurality of discrete zones of electromagnetic radiation along aprojection plane; and, at least a first imaging apparatus for imaging aplurality of reflections of said zones from at least one surfaceextending through said projection plane, said imaging apparatus lyingsubstantially in said projection plane; and at least a second imagingapparatus for imaging said plurality of reflections of said zones, saidimaging apparatus being spatially offset from said projection plane. 16.Optical measuring apparatus according to claim 15 further comprisingprocessing means for processing data output from said second imagingapparatus to discriminate between said zones and for processing datafrom said first imaging apparatus to obtain measurement datacharacterizing the spatial or dimensional characteristics of saidsurface.